The invention relates to a luminescent terbium-activated alkaline earth metal rare earth metal borate. The invention also relates to a luminescent screen provided with such a borate and to a low-pressure mercury vapour discharge lamp provided with such a screen.
Luminescent alkaline earth metal rare earth metal borates activated by Tb.sup.3+ whose fundamental lattice is defined by the general formula M.sub.3 Ln.sub.2 (BO.sub.3).sub.4 are known from British Patent Specification 1,453,991 (PHN 6798). In this formula M represents an alkaline earth metal and Ln represents a rare earth metal. All compounds defined by this general formula have an orthorhombic crystal structure. Upon excitation of such a luminescent borate by electrons, X-ray radiation or ultraviolet radiation, particularly short-wave ultraviolet radiation, the spectral distribution of the radiation emitted by the borate is that of the characteristic Tb.sup.3+ emission having a very steep, narrow peak (half-value width approximately 10 nm) at approximately 543 nm, with a number of side emissions.